The present invention is related to display apparatus and more particularly to display apparatus in which a display device, including transparent areas and areas capable of light diffusion, is illuminated from the back.
Generally, in such display apparatus as above mentioned, the light diffusable areas represent indicia to be displayed and the transparent areas represent the indicia-defining background. For such backlighted displays to be at all effective, an observer should receive light from the light diffusing areas, and simultaneously, to preserve contrast, should receive light of lesser intensity, or of different color, from the transparent areas.
One approach to providing this result is shown in U.S. Pat. No. 3,674,341, Hedman et al. Black or colored surfaces are disposed directly behind the display material and light from a source is directed obliquely onto the display material either by light baffles or light baffles in combination with mirrored surfaces. Some of the light striking the diffusing areas is redirected by forward scattering toward an observer in front of the display while light striking the transparent areas escapes from the device substantially unaltered.
Such an approach produces problems in some environments such as airplane cockpits. The primary disadvantage is attributable to light which escapes through the transparent areas of the device. Such direct transmission of source light into the cockpit is especially detrimental to the night vision of the aircraft operators. Other problems reside in the inefficient use of the light energy, which in an aircraft cockpit would result in inefficient use of aircraft electrical power and also would result in additional heat undesirably generated in the cockpit.
Another approach is set forth in U.S. patent application Ser. No. 377,487 assigned to the assignee of the present invention. (Said application issued as U.S. Pat. No. 3,863,246 on Jan. 28, 1975.) Therein, a prism is employed between the light source and the display. Louvered material is attached to the back of the prism for providing contrast and for controlling the direction of light rays impinging on the front display surface. In some embodiments, the light is prevented from striking the display surface at angles less than the critical angle of incidence. Thus, all undiffused light is internally reflected.
However, it is difficult to consistently achieve proper bonding between the prism and the louvered material. In addition, since louvers columnize light rays by eliminating or blocking out those rays which are not propagating in the desired direction, louvers are a fairly inefficient means of controlling light ray direction.
Therefore, it is an object of this invention to provide an improved backlighted display apparatus. It is a more specific object of this invention to provide a backlighted display apparatus with good contrast, which prevents blinding of an observer, and which overcomes the problems associated with baffle or louver light-control schemes.